Many filter devices each having a filter medium layer formed of granular filter media are used for solid-liquid separation and biotreatment equipment. The above filter devices perform surface filtration in which suspended solids in a liquid to be treated are mainly captured on a surface of the filter medium layer. In the surface filtration, the captured suspended solids are accumulated on the surface of the filter medium layer, and cause clogging of the filter medium layer even when the entire filter medium layer is not effectively utilized. For this reason, a filtration pressure rises in a short time and the filter media needs cleaning frequently. When flow passages (gaps between adjacent gains of the filter media) in the filter medium layer are narrow, poor balance in the filter medium layer makes surface filtration likely to occur. In depth filtration, suspended solids in a liquid to be treated are captured not only on the surface of the filter medium layer but also inside of the filter medium layer. Such depth filtration is less likely to undergo a rise in the filtration pressure in the filter medium layer and captures a large amount of solids per cycle. When flow passages, through which the suspended solids can pass, are formed deep inside the filter medium layer, the entire filter medium layer can be effectively utilized.
As an example of filter media included in the filter medium layer, Patent Literature 1 proposes a filter device in which a filter medium layer having a certain thickness is formed of indefinite form granular or fiber filter media being porous there inside, and suspended solids contained in a liquid to be treated are captured in the insides and on the surfaces of the filter media.
Patent Literature 2 proposes a filter device in which granular materials such as sand and anthracite, which are different in specific gravity and grain size, are formed into multiple layers to capture large suspended solids in large gaps formed by grains having a large effective diameter and to capture small suspended solids in small gaps formed by grains having a small effective diameter.
Also, Patent Literature 3 proposes a filtration method capable of maintaining filtration performance over a long period of time while suppressing filtration pressure rise by forming a filter medium layer by mixing two kinds of filter media different in grain size (a grain size a and grain sizes 3 to 5a) in appropriate amounts and thereby adjusting gaps between the filter media. In Patent Literature 3, for example, foam polystyrene filter media having a specific gravity of 0.1 and a grain size of 0.6 mm and having a specific gravity of 0.1 and a grain size of 2.8 mm are used as the two kinds of granular filter media. Patent Literature 3 describes a floating filtration method with less pressure loss for a long period of time and a large amount of SS to be captured, using a filter tank in a state where the two kinds of granular filter media are stirred and mixed without two kinds of floating filter medium grains separated from each other.
Patent Literature 4 proposes a depth filtration technology using cylindrical filter media with a high void ratio.
There has been known a sand filter device using both granular active carbon and garnet. For example, Patent Literature 5 discloses a filter device which performs downward-flow filtration process with a first filter medium layer filled with sand or garnet having a grain size of 0.45 to 0.8 mm, or both the sand and garnet in a multi-layer, and with a second filter medium layer filled with granular active carbon or anthracite having a grain size of 0.9 to 1.6 mm and formed on a support member above the first filter medium layer.
There has been known a filter device which performs solid-liquid separation and biotreatment using resin filter media, fiber filter media and the like. For example, as primary effluent treatment equipment using resin filter media, Patent Literature 6 discloses an upward-flow type solid-liquid separator in which a filter medium layer inside a treatment tank is filled with small cylindrical filter media each having a void ratio of 70% or more and a specific gravity of 1.0 or less.
As a flowable filler for use in an aeration chamber for biotreatment or the like, Patent Literature 7 discloses a water treatment filler which is made of synthetic resin having a specific gravity of 0.800 to 0.999, has a large surface area and appears to be approximately spherical.
As fiber filter media which separate suspended solids contained in a sewage, Patent Literature 8 proposes water treatment fiber filter media each made of a twisted filter medium produced by interlacing filaments with core and press yarns twisted together, and by performing heat treatment on the resultant yarns.